As used herein, the term GC/MS refers to a gas chromatograph (GC) interfaced to a mass spectrometer (MS). The term LC/MS refers to a liquid chromatograph (LC) interfaced to a mass spectrometer. The current practice in mass spectrometry is to have separate instruments for GC/MS and LC/MS operation. At least one manufacturer, Varian, Inc., manufactures a mass spectrometer that can be converted from atmospheric pressure LC/MS to a vacuum ionization GC/MS by breaking vacuum and interchanging ion sources. This approach suffers the disadvantages of being time consuming, requires breaking vacuum and is only applicable on the specific Varian instrument.
Atmospheric pressure ionization mass spectrometers (APIMS) instruments currently available lack flexibility. They are either configured to receive effluent from an up-stream gas chromatograph or from an up-stream liquid chromatograph, but cannot be easily changed to accept an alternate source of effluent. Typically, primary ions are formed at atmospheric pressure by initiation of a gaseous electrical discharge by an electric field or by electrospray ionization (ESI) as described in U.S. Pat. No. 6,297,499 (Fenn) and; U.S. Pat. No. 5,788,166 (Valaskovic). The primary ions in turn ionize the gas phase analyte molecules by either an ion-molecule process as occurs in atmospheric pressure chemical ionization (APCI), by a charge transfer process, or by entraining the analyte molecules in a charged droplet of solvent produced in the electrospray process. In the case of analyte being entrained in a charged liquid droplet, the ionization process is the same as in electrospray ionization (ESI) because the analyte molecules are first entrained in the liquid droplets and subsequently ionized.
Electrospray ionization (ESI) is a powerful method for producing gas phase ions from compounds in solution. In ESI, a liquid is typically forced from a small diameter tube at atmospheric pressure. A spray of fine droplets is generated when a potential of several thousand volts is applied between the liquid emerging from the tube and a nearby electrode. Charges on the liquid surface cause instability so that droplets break from jets extending from the emerging liquid surface. Evaporation of the droplets, typically using a counter-current gas, leads to a state where the surface charge again becomes sufficiently high (near the Raleigh limit) to cause instability and further smaller droplets are formed. This process proceeds until free ions are generated by either the evaporation process described above or by field emission that occurs when the field strength in the small droplets is sufficiently high for field evaporation of ions to occur. Molecules more basic than the solvent being used in the ESI process are preferentially ionized. Because ESI generates gas phase ions from a liquid, it is an ideal ionization method for interfacing liquid chromatography (LC) to mass spectrometry (MS). The power of ESI for the analysis of compounds as large and diverse as proteins won the 2003 Nobel prize in Chemistry for John Fenn. The combination of ESI with MS with liquid separation methods is extremely powerful analytically and results in large numbers of LC/MS instruments being sold each year.
Because ESI is most sensitive and most suitable for basic and polar compounds, most LC/MS instrumentation incorporates an alternative atmospheric pressure ionization (API) technique called atmospheric pressure chemical ionization (APCI). APCI was initially developed by Horning, et al. using 63Ni beta decay for ionization. See Horning, E. C.; Horning, M. G.; Carroll, D. I.; Dzidic, I.; Stillwell, R. N., New Picogram Detection System Based on a Mass Spectrometer with an External Ionization Source at Atmospheric Pressure. Anal. Chem., 1973. 45: p. 936-943. A discharge ion source has since replaced the 63Ni as the source of ionization. A discharge is generated when a voltage, typically applied to a metal needle, is increased to a range where electrical breakdown (formation of free electrons and ions) of the surrounding gas occurs (typically several thousand volts). The primary use of this ionization method has been as an ionization interface between liquid chromatography and mass spectrometry. See Dzidic, I.; Carroll, D. I.; Stillwell, R. N.; Horning, E. C., Comparison of Positive Ions formed in Nickel-63 and Corona Discharge Ion Sources using Nitrogen, Argon, Isobutene, Ammonia and Nitric Oxide as Reagents in Atmospheric Pressure Ionization Mass Spectrometry. Anal. Chem., 1976. 48: p. 1763-1768. This ionization method relies on evaporation of the liquid exiting the liquid chromatograph with subsequent gas phase ionization in a corona discharge. The primary ions produced in the corona discharge are from the most abundant species, typically nitrogen and oxygen from air or solvent molecules. Regardless of the initial population of ions produced in the corona discharge, diffusion controlled ion-molecule reactions will result in a large steady state population of protonated solvent ions. These ions in turn will ionize analyte molecules by proton transfer if the reaction is exothermic or by ion addition if the ion-molecule product is stable and infrequently by charge transfer reactions. While this technique tends to be more sensitive than ESI for low molecular weight and less polar compounds, it nevertheless is not sensitive for highly volatile compounds and those less basic than the LC solvent. Thus, neither atmospheric pressure chemical ionization (APCI) nor electrospray ionization (ESI) are good ionization methods for a large class of volatile and less polar compounds. For this reason, other ionization methods, such as photoionization have been applied to LC/MS to more effectively reach a subset of this class of compounds (See, for example U.S. Pat. No. 5,245,192, U.S. Pat. No. 6,646,256, U.S. Pat. No. 6,630,664, and published U.S. application US20030111598). Photoionization at atmospheric pressure uses an ultraviolet (UV) source for ionization of gas phase molecules. Typically, a plasma-induced discharge lamp that produces ultraviolet radiation in the range of 100-355 nanometers (nm) is used to generate ionization. Such a source, suitable for use with LC/MS, is available from Synagen Corporation, Tustin, Calif.
Thus, liquid chromatographs interfaced with the atmospheric pressure ionization methods of ESI and APCI are in common use and frequently the mass spectrometers associated with these ionization methods have advanced analytical capabilities such as MSn (MS/MS, MS/MS/MS, etc.) and/or high mass resolution and accurate mass analysis. However, LC/MS instruments do not effectively address a large class of important volatile and less polar compounds. Herein is described atmospheric pressure ionization for gas chromatographic effluents which is capable of ionizing a large portion of this compound class with high chromatographic resolution and high sensitivity using mass spectrometers designed for LC/MS applications.
Gas chromatography is commonly interfaced to mass spectrometers. The gas chromatograph is limited to volatile molecules but has higher resolving power than liquid chromatography based instruments. The gas chromatograph operates at atmospheric pressure and is interfaced to the mass spectrometry through a pressure drop device. Commonly, the pressure drop device is capillary tubing or a so-called ‘jet separator’, both of which limit the volume of gas entering the vacuum region of the mass spectrometer.
Gas chromatographs have been interfaced to API sources. A series of publications have appeared where the effluent from a gas chromatograph is ionized at atmospheric pressure using radioactive 63Ni as the source for production of negative ions. The most recent publication is Kinouchi, T.; Miranda, A. T. L.; Rushing, L. G.; Beland, F. A.; Korfmacher, W. A., J. High Resolution Chromatogr. & Chromatogr. Commun., 1990. 13(1): p. 281-284. The interface used in these experiments couple the GC to a 63Ni ion source of a specially built mass spectrometer, such as from Extranuclear Laboratories, Inc. (now ABB, Inc.) (See Siegal, M. W.; McKeown, M. C., J. Chromatogr., 1976. 122: p. 397) or a Finnigan-MAT 4000 (now Thermo Finnigan) (See Mitchum, R. K.; Korfmacher, W. A.; Freeman, J. P., An Atmospheric Pressure Ionization Source for a Finnigan-MAT 4000 Mass Spectrometer. Anal. Instrumentation, 1986. 15(1): p. 37-50). The publications, however, do not disclose any of the essential parameters that would allow transfer of the technology to modern atmospheric pressure instruments that have been designed for LC/MS applications. In addition, only negative ionization is discussed in the publications, a method limited to highly electronegative compounds.
A review paper by E. C. Horning, et al discusses both GC/APIMS and LC/APIMS ion sources (See Horning, E. C.; Carroll, D. I.; Dzidic, I.; Haegele, K. D.; Lin, S.-N.; Oertil, C. U.; Stillwell, R. N., Development and Use of Analytical Systems Based on Mass Spectrometry. Clin. Chem., 1977. 23(1): p. 13-21). This article shows diagrams of each ion source and refers back to two previous publications for details on LC/APIMS and on GC/APIMS. (Respectively see Carroll, D. I.; Dzidic, I.; Stillwell, R. N.; Haegele, K. D.; Horning, E. C., Atmospheric Pressure Ionization Mass Spectrometry: Corona discharge Ion Source for use in a Liquid Chromatography-Mass Spectrometry-Computer Analytical System. Anal. Chem., 1975. 47: p. 2369-2373 and see Dzidic, I.; Carroll, D. I.; Stillwell, R. N.; Horning, E. C., Comparison of Positive Ions formed in Nickel-63 and Corona Discharge Ion Sources using Nitrogen, Argon, Isobutene, Ammonia and Nitric Oxide as Reagents in Atmospheric Pressure Ionization Mass Spectrometry. Anal. Chem., 1976. 48: p. 1763-1768.
However, it is believed that there are no reports of an LC/APIMS source and a GC/APIMS source being interfaced to the same mass spectrometer or of a combined LC/APIMS and GC/APIMS source, or of interfacing a gas chromatograph to a mass spectrometer that is designed for LC/APIMS introduction. Nor have there been reports of switching between LC/MS and GC/MS operation in seconds as can be done with the present invention. In particular, the use of a dry purge gas to increase the types of compounds that can be ionized at atmospheric pressure has not been reported. Electrospray ionization has not been discussed in the literature in relation to GC/APIMS nor have the necessary conditions for effectively transporting compounds from the gas chromatograph to the atmospheric ionization region been discussed. No work has been reported on accurate mass measurement of atmospheric pressure GC/MS produced ions, or on GC/APIMS/MS or on GC/APIMS selected or multiple ion monitoring, all of which are techniques that are not readily available in most GC/MS instrumentation.
Commercial mass spectrometers have been manufactured that analyze gaseous compounds using corona discharge APCI, e.g. ABB, Inc., Extrel Quadrupole mass spectrometers, described in Ketkar, S. N.; Penn, S. M.; Fite, W. I., Real-time Detection of Parts per Trillion of Chemical Warfare Agents in Ambient Air Using Atmospheric Pressure Ionization Tandem Quadrupole Mass Spectrometry. Anal. Chem., 1991. 63: p. 457-459. and Sciex. mass spectrometers, described in Lave, D. A.; Thompson, A. M.; Loveft, A. M.; Reid, N. M., Adv. Mass Spectrom., 1980. 8B: p. 1480. and Reid, N. M.; Buckley, J. A.; Pom, C. C.; French, J. B., Adv. Mass Spectrom., 1980. 8B: p. 1843. Two patents (EP 0819937 A2 and U.S. Pat. No. 5,869,344) which disclose use of a Venture pump in combination with water vapor introduction for analysis of trace volatiles in air from sources such as breath and fragrances emulating from skin and clothing. Papers by L. Charles, et al and by G. Zehentbauer, et al have been published that reportedly improve on this method. (Respectively see Charles, L.; Riter, L. S.; Cooks, R. G., Direct Analysis of Semivolatiel Organic Compounds in Air by Atmospheric Pressure Chemical ionization Mass Spectrometry. J. Agric. Food Chem., 2000. 48: p. 5389-5395. and see Zehentbauer, G.; Kirck, T.; Teineccius, G. A., J. Agric. Food Chem., 2000. 48: p. 5389-5395.)
Pyrolysis with ionization of the gaseous pyrolysate has been reported, (see Snyder, A. P.; Kremer, J. H.; Mouzelaar, H. L. C.; Windig, W.; Taghizahed, K., Curie-point pyrolysis atmospheric pressure chemical ionization mass spectrometry: preliminary performance data for three biopolymers. Anal. Chem., 1987. 59: p. 1945-1951. while W. E. Steiner, et al has reported APCI of warfare agent simulants (see Steiner, W. E.; Clowers, B. H.; Haigh, P. E.; Hill, H. H., Secondary Ionization of Chemical Warfare Agent Simulants: Atmospheric Pressure Ion Mobility Time-of-Flight Mass Spectrometry. Anal. Chem., 2003. 75: p. 6068-6076.
A wafer thermal desorption system has been described for introducing samples into APIMS (in published US patent application US2002148974). Several patents (for example, JP2002228636, WO2002060565, U.S. Pat. No. 6,474,136, US2003092193, US2003086826, U.S. Pat. No. 6,032,513, U.S. Pat. No. 6,418,781, JP09015207, and JP06034616) discuss the use of GC and APIMS for the analysis and quantitation of trace gases such as hydrogen, oxygen, argon, carbon dioxide, carbon monoxide, freons, silanes, and other compounds that are gases at ambient temperature, primarily for the semiconductor industry.
Currently available mass spectrometers do not combine LC/MS and GC/MS in a single instrument without major source modification. The great majority of mass spectrometers are either designed for LC/MS operation or GC/MS operation, but not both. Many laboratories will have both GC/MS and LC/MS instruments available, but a growing number of laboratories have only LC/MS instrumentation. Therefore, it is desirable to devise an ionization source that allows commonly available LC/MS mass spectrometers to be interfaced to gas chromatographs. Such an instrument would extend the coverage of compounds that can be analyzed by currently available LC/MS instruments. Such an interface would have the additional advantage that the advanced capabilities common in LC/MS instruments, but not common in GC/MS instruments (e.g. techniques known to those practiced in the art such as cone-voltage fragmentation, MSn, high-mass resolution, accurate mass measurement) would become available to GC/MS analysis without purchase of new and expensive instrumentation. A gas chromatograph built into a probe that can be inserted into the standard LC/MS probe inlet would allow rapid switching between LC and GC/MS operation with little modification of the LC/APIMS ion source.